Project Summary Pericytes are specialized mural cells in the basement membrane of brain capillaries. Their contact and communication with the endothelium is critical for multiple aspects of vascular function, including control of microvascular blood flow and blood-brain barrier integrity. There is significant evidence that increased loss of pericytes occurs during Alzheimer's disease and Alzheimer's-related dementias, and that this loss causes accelerated degradation of microvascular integrity, leading to neuronal dysfunction. Preserving pericyte- endothelial contact may therefore improve cerebrovascular function in these neurodegenerative diseases. However, there remain fundamental gaps in knowledge on how the adult brain responds to and recovers from pericyte loss in vivo. We recently discovered that pericytes of the brain undergo a repair strategy to maintain coverage of the endothelium in the event of pericyte loss (Berthiaume et al. Cell Reports, 2018, 22(1):8-16). Pericytes can structurally remodel their far-reaching processes to invade endothelial regions that lack pericyte contact. The goal of this project is to investigate this novel facet of brain pericyte biology and its role in maintenance of capillary function. Our innovative approach will assess the effect of pericyte loss and repair in a completely physiological setting. We will use high-resolution, in vivo two-photon microscopy to image and selectively ablate pericytes, while assessing capillary hemodynamics, tissue oxygenation, and neural synaptic activity. This approach provides an exceptionally clear view of how the brain responds to pericyte loss, and the reparative responses that are mounted over days. In Aim 1, we will determine how the pericyte remodeling mechanism manages graded increases in severity of pericyte loss. We will examine the physiological consequence of this pericyte loss on capillary flow, structure and integrity, and determine whether the repair capacity is diminished with increasing age. In Aim 2, we will examine how pericyte loss alters the microstructure of tissue oxygen distribution and neuronal synaptic function using novel imaging probes. In Aim 3, we will determine whether pericyte remodeling is altered by activation or inhibition of PDGF-B/PDGFR?, a key signaling pathway for developmental recruitment of pericytes to their peri-endothelial niche. If successful, our aims will establish whether it is useful to restore pericyte coverage in conditions such as Alzheimer's disease and related dementias. We will obtain information on how selective pericyte loss in adult and aged brain affects the dynamics of capillary function. Finally, we will establish novel methods to quantify and manipulate pericyte remodeling, allowing the phenomenon to be studied broadly in other models of cerebrovascular disease.